Process for the chemical nickel-plating of non-metallic articles



United States Patent K 59,976 Int. Cl. B44d 1/06; C230 1/00 US. Cl. 117-130 7 Claims ABSTRACT OF THE DISCLOSURE Non-metallic articles purified by mechanical and/or chemical treatment, activated by means of an aqueous palladium salt solution, of which the palladium salt is reduced to palladium, and dipped in a pre-heated nickelplating bath, which contains nickeland hypophosphite ions and a buffer, are nickel-plated in the presence of nitrilo-trismethylene-phosphonic acid.

The present invention relates to a process for the chemical nickel-plating of non-metallic articles by means of a bath containing nickeland hypophosphite ions, and a complex former for stabilizing the bath and for accelerating the precipitation of nickel on the surface of the article.

A process for the chemical nickel-plating of articles treated to have a catalytic surface, has been described in German Patent 1,077,940, which process essentially comprises treating the articles by means of a bath containing nickeland hypophosphite ions. The term catalytic as used in this content is intended to mean that at least the surface material of the article is capable of catalyzing the oxidation-reduction reaction between the nickel ions and the hypophosphite ions, that causes the nickel to precipitate on the catalytic surface.

To ensure correct precipitation of nickel, the bath is primarily required to contain the reactants in specific proportions, i.e. the bath is required to contain:

Ni++ and H PO --ions in a quantitative ratio between i 0.25 and 1.60;

to contain T PO in an absolute concentration between 0.15 and 1.20 mols/liter;

and to have a pH-value between 4.3 and 6.8, which is maintained by means of a buffer present therein and by the addition of alkali during the reaction. Furthermore, to be active and useful for some prolonged time, the bath is required to contain so-called activating agents formed of saturated, aliphatic dicarboxylic acids having 3 to 6 carbon atoms, such as succinic acid. The reaction is ultimately required to be carried out at a temperature be tween 95 and 99 C. .As the applicability of the above process is limited to the chemical nickel-plating of articles having a catalytic" surface, it cannot simply be used for metallizing nonmetallic articles, as proposed in the present invention.

It has already been reported that the surfaces of nonmetallic articles can be treated to receive, for example, a metallic nickel plating. To this end, the article is first purified to activate its surface which is roughened thereafter, and pre-treated by means of a palladium salt solution, for example a palladium chloride solution. The palladium chloride is reduced to palladium and the article so activated is immersed thereafter in a bath containing nickeland hypophosphite ions. Reference is made to the process described in German published specification ice 1,182,015, which enables the surfaces, for example, of plastics, rubber, glass, wooden or ceramic material to be metallized. The palladium chloride solution suitable for surface activation preferably has a concentration of 35 milligrams per liter. When hypophosphite is the reducing agent, it is advantageous to dissolve it in the metallization bath in a concentration of 0.225 mol/liter.

In accordance with the process of German Patent 1,198,643, the palladium-activated article should preferably be nickel-plated first with the use of a dilute preliminary bath containing Ni++ and H PO in a molar concentration of about 0.25 and, after formation of a continuous nickel plating, the article should be treated further by means of a bath customary in nickel-plating. The preliminary nickel-plating bath is said to avoid the induction period usually needed for the nickel to precipitate on the dispersed palladium particles as the growthpromoting nuclei, and to improve the bond strength of the nickel plating. The preliminary nickel-plating bath may contain, for example, the following components:

Mol/liter NiSO -6H O -s 0.09 NEIHgPOg H 0.02 (NH SO 0.09

The preliminary bath used in carrying out the nickelplating should have a pH-value between 5.5 and 7 and a temperature between 10 and 30 C. To increase the nickelplating velocity, the bath may be mixed with an ordinary,

short-chain, saturated aliphatic monocarboxylic acid, such as acetic, butyric or valeric acid, which is added in salt form.

The disadvantage associated with the use of a preliminary bath in carrying out the nickel-plating is seen to reside in the relatively low precipitation velocity of the nickel on the surface of the article to be plated therewith, in the suggested temperature range of 10 to 30 C., so that a layer not thicker than 1 to 1.2 is produced per hour. In the event that the temperature is increased to a value higher than 30 C., the preliminary nickel-plating bath is found to undergo spontaneous self-decomposition with the resultant formation of colloidal nickel, and to become useless. h

The process of German Patent 1,077,940 mentioned above is not applicable to the direct nickel-plating'of nonmetallic articles, for example plastics. The reason is that thermoplastics, for example, undergo plastic deformation at a temperature of -99 C. needed for the nickel-plat lng bath, and that the precipitated layer of nickel would be found to have no more than a poor bond strength.

It has now been found that'the disadvantage associated with the earlier processes can be obviated in the present invention through the use of a nickel-plating bath. of customary composition, but stabilized by the addition of a specific complex former.

The present process of the present invention for the chemical nickel-plating of non-metallic articles which are (a) purified by mechanical and/or chemical treatment,

(b) activated thereafter by means of an aqueous palladium salt solution, of which the palladium salt is reduced to palladium, and (c) immersed in a preheated nickel-plating bath, which contains nickeland hypophosphite ions and I.

a buffer substance and is maintained at a pH-value' between 3 and 9, comprises nickel-plating the said articles in the presence of nitrilo-trismethylene-phosphonic acid, at a temperature between 30 and 70 C.

Reliable and optimum precipitation of nickel is ensured when the bath used for carrying out the nickel-plating contains the hypophosphite ions in a concentration of about 0.25 to 1 mol per liter, which is convenient. It is also 3 advantageous to provide for a molar ratio of Ni++-ions to H PO -ions of about 0.2 to 1.6 in the bath.

In accordance with a further feature of the present invention, the bath used for carrying out the nickel plating contains Ni++-ions and nitrilo-trismethylene-phosphonic acid in a molar ratio of about 0.7 to 2.0.

It has also been found that a bath maintained at a pH- value of 3 to 9, preferably to 6, causes the nickel to precipitate under especially favorable conditions. Phosphate buffer containing, per liter, 0.02 to 0.1 mol, preferable 0.067 mol Na HPO -6H O, and 0.002 to 0.01 mol, preferably 0.0067 mol KH PO has proved very suitable for maintaining the above pH-values. In addition to buffering the solution, the phosphate buffer is found to participate in the stabilization of the bath, at the pH-valnes indicated above. pH-values and temperatures higher than those specified above accelerate the precipitation of nickel, but they reduce the stability of the bath and impair the bond strength of the nickel plating, In the case of a bath having a pH-value higher than 7, it is possible further to increase the baths stability in conventional manner by the addition of a stabilizer, such as lead sulfide or lead acetate, which is used in a proportion of 2 milligrams per liter. In the following table there is indicated the bath temperature which should be conveniently used at a given pH-value, Where the bath is found to have a good stability and the nickel plating precipitated on the surface of the articles to be metallized is found to have the necessary bond strength.

TABLE pH-value: Temperature, C. 4 65 Under these conditions, the nickel is found to precipitate uniformly per unit of time on the surface of the article. In view of the fact that the nickel-plating bath composed as disclosed in this invention practically needs no induction period for the nickel to commence precipitation, it is clear that the articles dipped in the bath are completely metallized after no more than seconds. After a further 2 to 3 minutes, the precipitated nickel plating has a thickness of about 3 to 3.5 The nickel plating, which is really formed of a nickel-phosphorous alloy containing 5 to 15% phosphorus, adheres very tenaciously to non-metallic articles of which the surface has been activated in convenient manner, prior to the metallizationl The nickel plating precipitated by the process of the present invention can be used for reinforcing galvanic nickel, polished nickel and polished chromium coatings, or may receive a ductile copper coating as a first reinforcing coating. The steps of pre-treating the article to be plated in an attempt to activate its surface does not form part of the present invention. The activation can be achieved, for example, in the manner set forth in German Patent 1,182,015.

The process of the present invention can be used for the nickel plating of non-metallic substances or articles, such as polypropylene or glass or Bakelite or plastics obtained by the graft-polymerization of acrylonitrile, butadiene and styrene. 7

As compared with conventional baths used for nickelplating, the bath of the present invention offers the following advantages:

(1) The presence. of nitrilo-trismethylene-phosphonic acid means high precipitation velocity for the nickel at temperatures of the nickel-plating bath between 30 and 70 (3., so that non-metallic articles pretreated in convenient fashion, can be metallized more rapidly than heretofore.

(2) The bath is very stable and therefore easy to manipu:

late.

(3) Practically no induction period is needed for the nickel to commence precipitation.

(4) No preliminary bath is needed for carrying out the nickel-plating.

The bath used in carrying out the nickel-plating can be regenerated in customary manner by means of hypophosphite and nickel ions, for example by the process disclosed in German Patent 1,107,045. The regeneration is very easy to achieve as there is no need for cooling the bath liquid prior to the addition of the regenerating agent.

EXAMPLE Shaped plastics obtained by the graft polymerization of acrylonitrile, butadiene and styrene were chemically nickel-plated. Prior to being nickel-plated, they had been purified and degreased by means of an aqueous solution of a commercial wash-active substance, chemically roughened at 60 C. for 15 minutes with the use of chromo sulfuric acid and ultimately sensitized with an aqueous tin chloride solution in hydrochloric acid. Thereafter, the surface of the shapes had been activated by means of a PdCl solution in hydrochloric acid.

Following that conventional pre-treatment, the shaped plastics were nickel-plated by means of a nickel-plating bath prepared in accordance with the present invention, the bath containing:

Mol/liter NiSO 7H O 0.11 NaH PO H O 0.28 Nitrilo-trismethylene-phosphonic acid 0.12 Na HPO -2H O 0.067 KH PO 0.0067

The bath was adjusted by means of sodium hydroxide solution to have a pH-value of 5.5, and heated to 55 C. The shapes clamped to insulated holders, were dipped in the slightly agitated bath liquid. A continuous, welladhering nickel plating was found to have precipitated on the shapes within a period of 15 seconds, which, after a further 3 minutes, was found to have a thickness of 3.5,.

What is claimed is:

1. In the process for the chemical nickel-plating of non-metallic articles which are (a) purified, (b) activated thereafter by means of an aqueous palladium salt solution, of which the palladium salt is reduced to palladium, and (c) dipped in a pre-heated nickel-plating bath, which contains nickeland hypophosphite ions and a buffer and is maintained at a pH-value between 3 and 9, the improvement which comprises nickel-plating the said articles inthe presence of nitrilo-trismethylene-phosphonic acid, at'

a temperature between 30 and 70 C.

2. The process of claim 1, wherein the nickel-plating bath contains the hypophosphite ions in a concentration of about 0.25 to 1 mol per litter.

3. The process of claim 1, wherein the molar ratio of Ni++-ions to H PO -i0ns in the nickel-plating bath is between about 0.2 and 1.6.

4. The process of claim 1, wherein the molar ratio of Ni++-ions to nitrilo-trismethylene-phosp-honic acid in the nickel-plating bath is between about 0.7 and 2.0.

5. The process of claim 1, wherein the nickel-plating bath has a pH-value of 5 to 6.

6. The process of claim 1, wherein the nickel-plating bath is mixed with a phosphate buffer formed of 0.02 to 0.1 mol Na HPO -6H O and 0.002 to 0.01 mol KH PO per liter bath solution.

7. The process of claim 6, wherein the nickel-plating bath is buffered by means of 0.067 mol Na HPO -6H O and 0.0067 mol KH PO per liter bath solution.

(References on following page) 5 6 References Cited 3,427,197 2/1969 Lilker 117217 XR Baudrand etal.

12/1961 P Y- DAVID KLEIN, Primary Examiner 7/ 1964 Koretzky et a1.

9/1964- West. 5 U.S. c1. X.R.

6/1967 Zeblisky 106-1 106-1 1/ 1969 GulIa 106-1 

